1. Field of the Invention
The present invention relates to a semiconductor sensor chip used in a wide range of applications such as automobile, aircraft, medical service, measurement, and calibration, and to a production method for manufacturing the sensor chip. It also relates to a semiconductor sensor comprising the semiconductor sensor chip, and to a package for assembling the semiconductor sensor.
2. Description of the Related Art
An example of a conventional acceleration sensor chip disclosed in Japanese Patent No. 2551625 is shown in FIG. 1A and FIG. 1B. FIG. 1A is a perspective diagram, and FIG. 1B is a sectional diagram taken along line IBxe2x80x94IB of FIG. 1A. In this semiconductor acceleration sensor chip, a silicon single crystal is etched to form a support frame 1, weight parts 2a and 2b, a beam part 3a for connecting the weight part 2a and the weight part 2b, and beam parts 3b and 3c for connecting the weight part 2a, the weight part 2b and the support frame with each other. Gauge resistors 4a, 4b, 4c, and 4d are provided on the beam parts 3a, 3b, and 3c, and a Wheatstone bridge is formed of these gauge resistors. When an acceleration is exerted in a direction shown by the arrow in FIG. 1B, resistances of the gauge resistors are changed. The acceleration sensor chip measures the acceleration utilizing changes of the resistances.
In general, in the semiconductor acceleration sensor chip of this kind, a silicon substrate is deeply etched from the backside to form thick-walled weight parts of about 300 xcexcm to 400 xcexcm and thin-walled beam parts of about 10 xcexcm to 50 xcexcm. As the silicon substrate, a 4 inch wafer is often used. The reason for this is as follows:
Because the substrate is required to be deeply etched to form a thin beam part, a small wafer thickness is advantageous in terms of productivity due to the limitation of processing time. A wafer size which can be handled in the process with a thickness of about 300 xcexcm to 400 xcexcm, corresponding to the thickness of the beam part, is about 4 inches, and a larger wafer of 5 or 6 inches is substantially difficult to handle. Further, as shown in FIG. 1B, a wafer before dicing is formed with a number of thin-walled, low resonance frequency beam parts and is low in rigidity. A shock applied during dicing tends to generate a resonance phenomenon of the sensor part or the wafer itself, and there is a danger of an excessive displacement or stress applied to the beam parts. Consequently, the wafer size is limited because of this handling.
In the case of the above-described semiconductor acceleration sensor chip, a greater part of the cost is determined by chip size and wafer size. When acceleration sensor chips are produced with the same technical level, if the wafer size is large, a large number of chips can be processed in a single batch process, and the unit price of the chip is naturally reduced. However, in the above-described prior art, usable wafer size is limited, and cost reduction can only be achieved by reduction of the chip size. However, the chip size reduction is limited as it may reduce production yield. Further, in the future, with the trend to larger diameter semiconductor wafers, a decrease in supply of 4-inch wafers is anticipated. If such an acceleration sensor chip is achieved with larger-diameter wafers of 5 inches, 6 inches or the like, a beam part of 10 xcexcm to 30 xcexcm in thickness must be formed from a silicon substrate of about 600 xcexcm to 700 xcexcm in thickness, which not only increases the etching time but also leads to a reduced production yield.
Another example of a prior art acceleration sensor chip is one which is disclosed in Japanese Laid-Open Patent Application No. 8-248058.
The second prior art example will be described with reference to FIGS. 2A and 2B. FIG. 2A is a perspective diagram of the acceleration sensor chip. FIG. 2B is a schematic diagram showing the structure of a comb electrode unit as part of the acceleration sensor chip.
This acceleration sensor chip has a three-layered structure comprising a first layer (support plate) 10, a second layer 11 as an insulation layer on the first layer, and a third layer 12 coated thereon. For example, it comprises a SOI (silicon-on-insulator) or epitaxial polysilicon wafer (polysilicon as a third layer grown on a single crystal silicon substrate through an insulation layer).
The third layer 12 is provided with a displaceable first support body 13 separated from the first layer 10 and a non-displaceable second support body 16 which is connected with the first layer 10. The first support body 13 has a mass body 15 disposed at the center and a plurality of first plates 14 extending in a direction perpendicular to the mass body 15. The second support body 16 has two mounting parts 18 straightly disposed at both ends and a plurality of second plates 17 extending in a direction perpendicular to the mounting parts 18. The second layers 11 disposed at lower parts of the plurality of first plates 14 and the mass body 15 are removed by etching so that the first support body 13 is displaceable in parallel with respect to the surface of the first layer 10.
Further, the plurality of first plates 14 and the plurality of second plates 17 respectively form comb electrodes, which, when the displaceable mass body displaces in a direction perpendicular to the first plate 14, measure an acceleration by utilizing a change in capacitance between the first plate 14 and the second plate 17. Still further, a conductor 19 for conducting these comb electrodes to an external circuit is electrically insulated from the first layer 10 by the second layer (insulation layer) 11, and further electrically insulated from the third layer 12 by a cutout 20.
In the capacitive type acceleration sensor chip using comb electrodes of this type, in order to increase the change in capacitance to affect an increase in sensitivity, it is necessary to form a structure with a decreased rigidity of a movable electrode (first plate 14). There are two factors that cause variations in sensitivity when such a sensor is constructed. A first factor is a variation in rigidity of the movable electrode (first plate 14) that is dependent on the production precision, and where the sensitivity is small when the rigidity is high. A second factor is the variation of gap between the movable electrode (first plate 14) and a fixed electrode (second plate 17), where the sensitivity decreases as the gap increases.
With respect to the first sensitivity variation factor, in general, production methods such as wet etching, RIE (Reactive Ion Etching), plasma etching and the like are used in process for producing the gap between the movable electrode and the fixed electrode and in the process of producing the support part of the movable electrode. With these production methods, since etching speed in a depth direction varies depending on the processing width, a variation occurs in the processing shape depending on the width of etching pattern. To prevent this, it is necessary to make a complex mask design in consideration of the etching speed which varies for every pattern width, resulting in a complicated process.
The second sensitivity variation factor will now be described in detail. In a sensor chip using a wafer in which polysilicon is formed as a third layer through an insulation layer on a single crystal silicon substrate or a SOI wafer, the second layer comprising an insulation layer, such as SiO2, between the first layer and the third layer and a passivation film for protecting circuits on an upper surface of the third layer are formed. As a result, there is a loss of balance in the internal stress between a surface on the side where the second and third layers are disposed on the first layer which controls the rigidity of wafer, and the opposite back surface, resulting in a warped wafer. Therefore, there is a problem in that due to such a warping of wafer, a strain occurs in the sensor structure formed on the third layer, thereby, there is variation in the gap between the movable electrode and the fixed electrode constituting the comb electrodes, for example, of the capacitive type sensor chip. Yet further, there is another problem in that in an initial state before measurement when such a detected physical amount is not yet generated, generation of a strain results in an increase in offset, which requires a complicated correction circuit.
Further, in the acceleration sensor chip, after the insulation layer is etched to form a number of sensor chips, in a subsequent process such as a dicing process to divide it into discrete chips, there is a problem in that foreign matter may enter the gaps between the comb electrodes. Also, static electricity generated during sensor operation may attract foreign matter from other packaged parts to the sensor part. Depending on the size of entering foreign matter, operation of the comb electrodes may be disturbed. Even when the size of the entering foreign matter is small enough that the matter does not disturb the operation of the comb electrodes, depending on the characteristics of the entering foreign matter, capacitance between the comb electrodes may be varied. Still further, there is another problem in that, when an epitaxial polysilicon wafer is used, since polysilicon is produced by a film forming apparatus such as a CVD apparatus, even if the same in outer dimensions, a deviation occurs in mechanical characteristics such as internal stress or breakdown stress, resulting in degraded reliability of the sensor chip.
The sensor chip is incorporated in a package 60, forming a semiconductor sensor. FIG. 3 shows an example of a prior art semiconductor sensor. In this prior art example, an acceleration sensor chip 50, for detecting an acceleration in a direction 70 perpendicular to the chip surface, is mounted on a printed circuit board 80 so that the perpendicular direction of the chip surface is correctly in line with the direction 70 of the acceleration. More specifically, a package 60 incorporating the acceleration sensor chip 50 is fixed with a sensor retaining pin 91 to a high-rigidity substrate 90, and the high-rigidity substrate 90 is mounted on the printed circuit board. Package terminals 61, electrically connected with input/output terminals (not shown) of the acceleration sensor chip, are connected to terminals 81 of the printed circuit with wiring 82. A similar construction to the semiconductor acceleration sensor shown in FIG. 3 is described in Japanese Patent Application Laid-open No. 8-94663 (1996) (U.S. patent application Ser. No. 08/189,948).
The sensor chip illustrated in FIG. 1A and FIG. 1B, for example, is used as the acceleration sensor chip 50. As for the semiconductor sensor illustrated in FIG. 3, it is possible to obtain an output according to the acceleration generated in the direction 70 perpendicular to the surface of the acceleration sensor chip 50.
However, the above-described prior art acceleration sensor has the following problems:
Because the acceleration sensor package 60 is mounted on the printed circuit board 80 through the high-rigidity substrate 90, the area required for mounting is increased, and the entire acceleration measuring system, including the printed circuit board 80, is increased in size.
Mechanical vibration of the wiring 82 transmits vibration to the sensor package, resulting in mechanical noise. Further, since the wiring 82 is located in a three-dimensional space, it tends to cause an induction noise from outside the sensor chip.
A process for fixing the package 60 to the high-rigidity substrate 90, a process for wiring from the package 60 to the printed circuit board 80 and the like are required. These processes are difficult to automate, resulting in increased assembly cost.
A first object of the present invention is to solve the above-described problems in prior art conventional semiconductor sensor chips.
Specifically, a first object of the present invention is to solve the following problems:
(i) in an acceleration sensor chip using a simple piece of single crystal silicon wafer, use of a thick, large-diameter wafer is difficult,
(ii) in a capacitive type acceleration sensor chip using SOI wafer or epitaxial polysilicon wafer,
a) increasing the sensor sensitivity is difficult,
b) in the dicing process after removing the insulation layer, foreign matter may enter the sensor structure,
c) variations of sensitivity and offset are large because of the strain of sensor caused by the warping of wafer.
d) detection capacity is changed by foreign matter, and
e) the sensor structure possesses less reliable mechanical characteristics.
A second object of the present invention is to provide a semiconductor sensor and a semiconductor sensor package which solves the problems of conventional semiconductor sensors mentioned above by reducing the mounting area, preventing generation of mechanical and induction noises due to wiring, and lowering the mounting cost.
To achieve the first object, in accordance with one aspect of the invention, there is provided an acceleration sensor chip comprising a support frame part, and a sensor structure including at least one displaceable weight part, and a beam part for connecting the weight part to the support part. The support frame part and the sensor structure are formed on a silicon substrate through an insulation layer, wherein the insulation layer between the sensor structure and the silicon substrate is removed. The beam part comprises a plurality of sets of beams which are parallel to each other, the weight part is connected to the support frame part by the plurality of sets of parallel beams, and at least two semiconductor strain gauges are formed on the surface of at least one set of the plurality of sets of parallel beams.
In this case, there is preferably one weight part, the plurality of sets of parallel beams are protrudingly formed to four corner parts of the weight part, and four semiconductor gauges are respectively formed on the surfaces of the plurality of sets of beams, thus forming a Wheatstone bridge. In an alternative case, there are preferably two weight parts, the plurality of sets of parallel beams are formed between the two weight parts and the support frame part and between the two weight parts, at least one semiconductor strain gauge is formed on the respective surface of (a) at least one of beams (b) between one of the two weight parts and the support frame part of the plurality of sets of parallel beams, at least one of beams between the other of the two weight parts and the support frame part, and (c) a beam between the two weight parts, and a Wheatstone bridge is formed of the semiconductor strain gauges.
Further, preferably, the thickness of the beam part is smaller than that of the weight part.
Still further, in the acceleration sensor chip according to the present invention, a sensor structure comprises a displaceable weight part having a magnetic thin film formed on the surface and a beam part for connecting the weight part to the support frame part, the sensor structure being formed on a silicon substrate through an insulation layer, the insulation layer between the sensor structure and the silicon substrate removed, and, on the support frame part on the periphery of the weight part, a coil is formed to surround the weight part.
Further, according to another aspect, there is provided an acceleration sensor chip comprising a support frame part, and a plurality of sensor structures including displaceable weight parts respectively having magnetic films formed on the surfaces, and beam parts for connecting the weight parts to the support frame part, the support frame part and the sensor structures being formed on a silicon substrate through an insulation layer, wherein the insulation layer between the plurality of sensor structures and the silicon substrate is removed, coils are respectively formed surrounding the weight parts on the support frame part on the periphery of the respective weight parts, and the plurality of coils are connected in series.
Here, it is preferable that a plurality of sensor groups comprising the respective plurality of sensor structures and the plurality of detection coils connected in series, the sensor groups differing in numbers of the sensor structures, and the detection coils are formed on a same semiconductor chip.
In the above-described acceleration sensor chip, it is desirable to further provide a means for performing a self diagnosis, and an amplifier circuit and a digital adjustment circuit are formed on the semiconductor chip on which the acceleration sensor chip is formed.
According to a yet further aspect, there is provided an angular acceleration sensor chip comprising a first sensor group including a first support frame part, and a plurality of first sensor structures comprising displaceable first weight parts having magnetic thin films formed on the respective surfaces and first beam parts for connecting the first weight parts to the first support part, the first support frame part and the first sensor structures being formed on a silicon substrate through an insulation layer, wherein the insulation layer between the plurality of first sensor structures and the silicon substrate is removed, first detection coils are respectively formed surrounding the first weight parts on the first support frame part on the respective periphery of the first weight parts, and the plurality of first detection coils are connected in series;
a second sensor group including a second support frame part, and a plurality of second sensor structures comprising displaceable second weight parts having magnetic thin films formed on their respective surfaces and second beam parts for connecting the second weight parts to the second support frame part, the second support frame part and the second sensor structures being formed on the silicon substrate through an insulating layer, wherein the insulating layer between the plurality of sensor structures and the silicon substrate is removed, second detection coils are respectively formed surrounding the second weight parts on the second support part on the respective periphery of the second parts, and the plurality of second detection coils are connected in series, the first and second sensor groups being formed on a same semiconductor chip;
the first sensor group and the second sensor group are equal in number of sensor structures, and the first sensor group and the second sensor group are disposed symmetrically about a detection axis as an axis of symmetry,
the first and second detection coils of the first and second sensor groups form closed loops so that currents flowing through the plurality of first and second detection coils of the first and second sensor groups flow in the same direction when an angular acceleration generates about the detection axis. This embodiment may further comprise means for amplifying signals from the plurality of first and second detection coils and means for integrating outputs from the plurality of detection coils to output an angular acceleration signal.
According to a yet further aspect, there is provided an acceleration sensor chip characterized in that a third layer is formed on a first layer of a support substrate through an insulating second layer, the third layer has a sensor structure, and the second layer between a detection surface of the sensor structure and the first layer is removed, and, a beam part having a detection device, and a weight part having a plurality of cutouts of a same width formed over the entire surface are provided on the detection surface of the sensor structure with the second layer removed.
Here, it is preferable that a film of a material having a smaller thermal expansion coefficient than the material of the first layer be formed on the backside of the first layer.
Further, it is desirable that the same width of the plurality of cutouts formed on the sensor structure be a width of 2 xcexcm or less.
Still further, as a substrate comprising the first layer, the second layer and the third layer, a silicon-on-insulator substrate may be used, or a substrate having polysilicon formed as the third layer on a single crystal silicon substrate through an insulation layer be used.
According to a yet further aspect, there is provided a production method of an acceleration sensor chip of the following processes.
Specifically, the production method of the acceleration sensor chip is characterized by comprising:
(a) a step for preparing a SOI wafer comprising a silicon substrate, a SiO2 layer and a silicon thin film;
(b) a step for ion implanting a dopant at a position corresponding to a semiconductor strain gauge of the silicon thin film to form a diffusion resistor, and forming devices necessary for circuit construction on the silicon thin film;
(c) a step for providing a protective film on the entire surface of the wafer, opening a through hole penetrating the silicon thin film by patterning and etching, and forming a weight part and a beam part connecting to a support frame part remained on the periphery;
(d) while maintaining the protective film, as is, for forming the through hole, a step for removing by wet etching the SiO2 layer under the weight part and the beam part;
(e) a step for removing the protective film, and coating a resist over the entire surface of the wafer;
(f) a step for forming a slit by dicing for dividing the chip while maintaining a small thickness of the wafer;
(g) a step for removing by ashing the resist on the wafer by an O2 plasma; and
(h) a step for dividing the chip by concentrating a stress on the slit.
According to a yet further aspect, there is provided a production method of an angular acceleration sensor chip of the following processes.
Specifically, the production method of the angular acceleration sensor chip is characterized by comprising:
(a) a step for preparing a SOI wafer comprising a silicon substrate, a SiO2 layer and a silicon thin film;
(b) a step for ion implanting a dopant at a position corresponding to a semiconductor strain gauge of the silicon thin film to form a diffusion resistor, forming a magnetic thin film at a position corresponding to a weight part, forming a detection coil surrounding the magnetic thin film, and forming devices necessary for circuit construction on the silicon thin film;
(c) a step for providing a protective film on the entire surface of the wafer, opening a through hole penetrating the silicon thin film by patterning and etching, and forming a beam part connecting to the weight part and a support frame part remained on the periphery;
(d) while maintaining the protective film, as is, for forming the through hole, a step for removing by wet etching the SiO2 layer under the weight part and the beam part;
(e) a step for removing the protective film, and coating a resist over the entire surface of the wafer;
(f) a step for forming a slit by dicing for dividing the chip while maintaining a small thickness of the wafer;
(g) a step for removing by ashing the resist on the wafer by an O2 plasma; and
(h) a step for dividing the chip by concentrating a stress on the slit.
According to a yet further aspect, there is provided a production method of an acceleration sensor chip of the following processes.
Specifically, the production method of the acceleration sensor chip for constructing a sensor structure on a third layer provided on a first layer of support substrate through an insulating second layer, characterized by comprising:
a first step for forming a plurality of cutouts of a same width on the third layer to form a detection surface of the sensor structure having a beam part and a weight part for displacing the beam part which are separated from each other;
a second step for filling the plurality of cutouts of the same width of the sensor structure with a sealing agent to flatten the surface of the third layer including the sensor structure;
a third step for forming a circuit part connected electrically to the sensor structure in the periphery of the surface-flattened third layer; and
a fourth step for removing the sealing agent filled in the plurality of cutouts of the same width and removing the second layer located beneath a detection surface of the sensor structure to make the beam part and the weight part provided on the detection surface of the sensor structure displaceable.
The above acceleration sensor chip production method may further have a fifth step for coating a protective film on the surface of the third layer including the sensor structure after the fourth step, forming a slit in the protective film-coated third layer, and performing dicing, and a sixth step for removing the protective film of the third layer after dicing.
Further, in any one of the first step to the fourth step of the acceleration sensor chip production method, a film with a smaller thermal expansion coefficient than the material of the first layer may be formed on the backside of the first layer.
Further, in the above acceleration sensor chip production method, the same width of the plurality of cutouts formed on the sensor structure may be a width of 2 xcexcm or less.
Still further, in the above acceleration sensor chip production method, as a substrate comprising the first layer, the second layer and the third layer, a silicon-on-insulator structure substrate may be used, or a substrate having polysilicon formed as the third layer on a single crystal silicon substrate through an insulation layer be used.
In accordance with the present invention, which attains the second object, there is provided a semiconductor sensor comprising a semiconductor sensor chip for detecting a physical value applied in a direction perpendicular to the surface of the chip and a package for incorporating the semiconductor sensor chip. In the package, a main surface for mounting the semiconductor sensor chip is formed to have a predetermined angle with respect to the surface of a printed circuit board for mounting the package, the main surface is provided with a plurality of terminals along two opposite sides thereof for connecting with input/output terminals of the semiconductor sensor chip, a bottom surface perpendicular to the main surface is provided with a plurality of pins respectively formed along the two sides parallel to the main surface, which plurality of pins are inserted into mounting holes formed in the printed circuit board, the plurality of terminals and the plurality of pins are electrically connected, and the input/output terminals of the semiconductor sensor chip mounted on the main surface are electrically connected with the plurality of terminals of the package.
In this case, the main surface for mounting the semiconductor chip is formed substantially perpendicular to the surface of the printed circuit board for mounting the package.
The semiconductor sensor chip may be a semiconductor acceleration sensor chip.
The semiconductor acceleration sensor chip may be any one of the above described acceleration sensor chips used to attain the first object.
Also, the semiconductor acceleration sensor chip may be the above described angular acceleration sensor chip used to attain the first object.
The semiconductor sensor package according to the present invention is a package for incorporating a semiconductor sensor chip characterized in that a main surface for mounting the semiconductor chip is formed at a predetermined angle with respect to the surface of a printed circuit board mounting the package, the main surface is provided with a plurality of terminals along two opposite sides thereof for connecting with input/output terminals of the semiconductor sensor chip, a bottom surface perpendicular to the main surface is provided with a plurality of pins respectively formed along the two sides parallel to the main surface, which plurality of pins are inserted into mounting holes formed on the printed circuit board, and the plurality of terminals and the plurality of pins are electrically connected along two side surfaces sandwiching the main surface.
Here, the main surface for mounting the semiconductor sensor chip can be formed substantially perpendicular to the surface of the printed circuit board mounting the package.
The wiring for connecting the plurality of terminals and the plurality of pins is preferably buried in the package.
The above and the other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.